In recent years, solar or photovoltaic (PV) batteries composed of PV cells assembled in PV modules, have been attracting attention as the next generation of clean energy sources. Applications ranging from building to electronic have been developed. The structure of the photovoltaic battery unit is based on a composition of the following elements: a high light transmission material, photovoltaic battery module, filled resin layer and back sheet laminate. The assembled panels can be integrated on building rooftops, used in solar farms and used for electric/electronic parts. The thermoplastic resin film is used as part of the back sheet laminate of the photovoltaic battery.
With reference to the thermoplastic resin sheet (for photovoltaic batteries), the durability in the natural environment, especially regarding hydrolysis resistance, light stability and weatherability, is required.
The conventional biaxially oriented polyester polymer sheets lack in hydrolysis and UV resistance after ageing, which are the main requirements to environmental conditions resistance. They also lack in opacity which masks defects caused by lamination of back sheet.
Furthermore, the improvement in electrical transduction efficiency of sunlight within photovoltaic batteries is also required. For this purpose, the light travelling from the front side of photovoltaic battery and reflected back by the back sheet laminate of photovoltaic batteries is converted into electricity. EP2221336 presents an opaque black film which does not give a satisfying maintenance ratio of reflectivity. Moreover black films do not answer the market requirements for white back sheets. Therefore, white polyester films are also required.
The use of chain extenders for improving the hydrolysis resistance of polyethylene terephthalate films is widely described in prior art (introduction of epoxidized fatty acid glyceride is described in U.S. Pat. No. 7,241,507B2, introduction of carbodiimide or oxazoline is described in U.S. Pat. No. 6,855,758B2 and EP2062938A1, introduction of phosphate compounds is described in JP2009256621A2 for example . . . ). Nevertheless, it is well known that chain extenders are not significantly used for industrial PET films production because they have tendency to induce gel formation, filtration issue, gas formation, yellowness.
To cope with the hydrolysis resistance issue, EP-1995789-A1 provides a polyester resin sheet for photovoltaic batteries which enhances the electrical transduction efficiency of photovoltaic batteries by improving the hydrolysis and heat resistance and further enhancing the screening potential. In addition, it provides a photovoltaic battery formed by using the same from the background of the related art. This polyester resin sheet for photovoltaic batteries, comprising a polyester resin layer formed by using one or more layers having a number average molecular weight of 18500 to 40000, in which at least one or more layers having 5 to 40% by weight of titanium dioxide is formed on the polyester resin layer, wherein a light transmittance at wavelengths of 300 to 350 nm is 0.005 to 10%, a relative reflectance is 80% or more and 105% or less, an apparent density is 1.37 to 1.65 g/cm3. In this polyester resin sheet for photovoltaic batteries, the thickness of the layer having 5 to 40% by weight of titanium dioxide is 7 to 100% of the total thickness of the polyester resin layer. This sheet is for example a three layered symmetrical structure: wherein the two outer layers excluding the core layer, contain TiO2 particles in a total amount of 5.5 to 40% w/w.
EP-1995789-A1 describes use of TiO2 in the outer layers. An additional drawback of the biaxially oriented white polyester polymer films is the occurrence of peeling in the laminate caused by a weak adhesion between PET film and the other layers of the back sheet laminate due to presence of inorganic particles in outer layers of PET film. Particles in outer layer also induce chalking phenomenon after ageing.